We are interested in understanding transcriptional regulation during early oogenesis, specifically the post-mitotic stages, and the transition from prenatal to postnatal ovary, when clusters of oocytes begin to form into primordial follicles. Early stages of ovarian follicle formation, beginning with the breakdown of germ cell cysts, formation of primordial follicles and transition to primary and secondary follicles, are critical in qetermining the reproductive life span and fertility. Large oocyte loses occur during these early stages of oocyte development, and unknown pro-survival factors stabilize such loss as oocytes arrest in meiotic prophase I, and primordial follicles form. Transcription of numerous germ cell specific genes, necessary and essential for follicular development and early embryogenesis, is initiated during these early stages of follicle formation. We discovered that LIM homeodomain gene, LhxB, is preferentially expressed in oocytes and critical in oocyte differentiation. In addition, multiple genes preferentially expressed in the oocyte, such as Kit are down-regulated in LhxB deficient oocytes. In Specific Aim 1, we will test the hypothesis that LhxB directly regulates numerous oocyte-specific genes including oocyte-specific transcriptional regulators. We hypothesize that LHX8 directly regulates expression of Kit, an essential regulator of primordial follicle formation. We will also perform chromatin immunoprecipitation with anti-LHX8 antibodies to determine genomic regions that LHX8 directly binds. In addition, we will study the effects of LhxB deficiency on the embryonic gonadal transcriptome and determine the onset of molecular pathology. In Specific Aim 2 we will test the hypothesis that the two LIM domains in the LHX8 protein interact with other oocytespecific transcriptional regulators as well as LIM domain binding proteins. LhxB represents the first oocyte-specific LIM homeodomain gene with critical functions during early oogenesis. Our proposed studies will provide insight into the mechanisms of LhxB action and further elucidate genetic pathways that govern oogenesis. It is likely that LhxB pathway or its co-regulators playa role in human premature ovarian failure.